The present invention is generally related to a roadway communication system wherein a plurality of road antennas are disposed along a road in a manner to define cells on the road, thereby providing mobile communications between a stationary station and a mobile station.
Demand for communications between the road administration and ground vehicles tends to grow even more in the future. Particularly, drivers on highways need frequent information exchanges between the stationary station and the mobile stations in order to drive under less load and to prevent accidents. A developed form of such a system is exemplified by an automatic driving system with extensive provision of various sensors and cameras on both the road and vehicle for close communications between the stationary station and vehicle (see, for example, Japanese Unexamined Patent Publication No.8-241495(1996).
The consideration of future expansion of the automatic driving system dictates the need to construct a driver support system (hereinafter referred to as xe2x80x9croadway communication systemxe2x80x9d) based on the communications with vehicles. Preparatory to the construction of such a system, communication areas (cells) must be provided over the roads.
It may be contemplated to lay leakage coaxial cables along the roads, which involves a large scale cable-laying work. Besides, the leakage coaxial cables need be embedded at a relatively great depth in the roads and hence, the wave energy presents a disadvantageously small range with respect to a transverse direction of the lane.
In contrast, as shown in FIG. 28, a system with road antennas 120 disposed at given space intervals along the road for communications permits one road antenna 120 to define a relatively broad cell 121. In this case, the road antennas are each connected to a respective control station of the road administration via an optical fiber, coaxial cable or the like.
(A) In a case where the road antennas are disposed, a large vehicle approaching a small vehicle may sometimes cut in a line of sight of the small vehicle. FIG. 29 illustrates a particular state where the small vehicle is in a wave blocking area of the large vehicle. The microwaves and millimeter waves at high frequencies have small diffraction angles and hence, are prone to be blocked. This results in a breakdown of roadway communications.
It is therefore, an object of the invention to provide a roadway communication system accomplishing seamless communications between the stationary station and the mobile station.
(B) The installation of the road antennas generally entails inter-carrier interference or inter-symbol interference associated with the occurrence of delayed multipath waves, causative factors of which are structures near the road or plural vehicles in the cell which reflect the waves. When the inter-symbol interference occurs, a bit error rate is not improved even if the waves are received at high reception levels. This leads to a so-called floor error.
In the mobile communication system based on a single carrier, a receiver is generally equipped with an equalizer having inverse characteristics of those of a transmission line thereby to eliminate the effect of the inter-symbol interference associated with the delayed multipath waves.
Unfortunately, the automobile travels through the cell at such high speeds that the radio frequency energy field presents too sharp fluctuations per unit time for the equalizer to cope with the calculations. Thus, it is impossible to transmit signals at less than a given transmission error rate. Additionally, a large-scale hardware is required for implementing the equalizer, which results in great power consumption.
On this account, the invention has an object to provide a roadway communication system capable of preventing the inter-carrier interference and inter-symbol interference for accomplishing stable communications between the stationary station and the mobile station.
(1) A roadway communication system according to the invention for achieving the above object has an arrangement wherein a plurality of road transmission antennas are disposed at different places along a road, each radiating the same cell with electromagnetic waves carried at the same frequency and containing the same content, and wherein a vehicle mounted device receiving the waves from the road transmission antennas performs antenna pattern diversity reception (Claim 1).
According to the invention, the plural road transmission antennas radiate the waves based on signals modulated with data of the same content. In this case, the road transmission antennas each have a specific directivity (including non-directivity) and therefore, the waves from the road transmission antennas are incident on the vehicle along different directions. Accordingly, when an incoming wave in one direction of the vehicle is blocked by a large vehicle, the wave incident on the vehicle in another direction becomes relatively higher in reception level than the blocked wave. Hence, the vehicle reception antennas performing the diversity reception can provide communications between the road transmission antennas and the vehicle mounted device even when one of the waves is blocked.
Within the same cell, seamless communications are ensured between the stationary station and the vehicle because the radiation to the vehicle is provided in plural different directions. This permits every vehicle to receive road traffic information seamlessly and hence, the inventive roadway communication system is best-suited to the automatic driving system, as well.
The diversity reception may be performed based on the reception level of each directive wave received by the vehicle reception antenna (Claim 2).
The reception level of a wave received by the vehicle reception antenna normally increases in value as the vehicle approaches a road transmission antenna radiating the wave. In a case where a wave to be received at the highest reception level is blocked by the large vehicle cutting in the transmission path thereof, for example, a reception level of another wave from a different road transmission antenna may become relatively higher than that of the former wave. In consideration of this event, an arrangement may be made such that the reception levels of the waves received by the vehicle reception antennas are directly detected and the directivity of wave to be received is switched. Thus are attained more preferable communications.
The diversity reception may be accomplished by either of the following operations: (a) an operation of switching or combining the signals which were received by the vehicle reception antennas and are to be decoded; and (b) an operation of switching or combining the codes which were received by the vehicle reception antennas and then decoded (Claim 3).
The vehicle reception antennas may be an array antenna, whereas the vehicle mounted device may further comprise reception-signal detection means for detecting a reception level or phase of the wave received by each of the vehicle reception antennas, so that the diversity reception means may perform the diversity reception using information on the reception level or phase detected by the reception-signal detection means (Claim 4).
This configuration assumes a case where the array antenna or adaptive array antenna is applied to the vehicle reception antennas. The phase control of the antenna provides a desired directivity.
If an optical fiber radio signal transmission system is used as a transmission system for supplying the signals to the transmission antennas (Claim 5), no need exists for each road transmission antenna to have a signal transmission unit with a frequency converter. This results in a simplified configuration of the road transmission antenna.
Orthogonal Frequency Division Multiplex (OFDM) modulation technique in which a guard time is provided at each symbol may be used as a data modulation technique (Claim 6). OFDM system resisting multipath effect is preferably applied to the inventive arrangement wherein the waves incident in different directions are received. Particularly, the provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
The signal transmission from the plural road transmission antennas to the same cell involves fear that a fractional difference in carrier frequency may occur between the transmission stations. It is known that OFDM is more susceptible to the degradation of transmission quality than other transmission systems. The optical fiber radio signal transmission system provides a highly effective and economical solution to this problem, ensuring that the carrier frequencies from the stations are completely matched.
According to the invention for achieving the above object, a roadway communication system comprises a plurality of road reception antennas for receiving electromagnetic waves radiated from a vehicle mounted device in different directions, the plural road reception antennas being disposed in a manner to provide directivity to the same cell and adapted to perform diversity reception based on the signals received the road reception antennas(Claim 7).
In the roadway communication system, the vehicle radiates the waves in different directions thereby providing data to the stationary station. The plural road reception antennas are so disposed as to provide directivity to the same cell white the site diversity reception is performed using the signals received by the respective road transmission antennas. As a result, the stationary station can positively receive the waves despite the multipath effect on the respective radio frequency energy fields of the road reception antennas.
The diversity reception may be performed based on the reception level of the wave received by the road reception antenna (Claim 8).
In a case where a wave to be received at the highest reception level is blocked by the large vehicle cutting in the transmission path thereof, for example, a reception level of another wave from a different road transmission antenna may become relatively higher than that of the former wave. In consideration of this event, an arrangement may be made such that the reception levels of the waves received by the road reception antennas are detected and the reception antenna to receive the wave is changed. Thus are attained more preferable communications.
The diversity reception may be accomplished by either of the following operations: (a) an operation of switching or combining the signals which were received by the road reception antennas and are to be decoded; and (b) an operation of switching or combining the codes which were received by the road reception antennas and then decoded (Claim 9).
If an optical fiber radio signal transmission system is used as a transmission system for receiving the signals from the road reception antennas (Claim 10), no need exists for each road reception antenna to include a signal transmission unit with a frequency converter. This results in a simplified configuration of the road reception antenna.
Orthogonal Frequency Division Multiplex (OFDM) modulation technique may be used as a data modulation technique (Claim 11). OFDM system resisting multipath effect is preferably applied to the inventive arrangement wherein the waves from the vehicle on road are received in an environment having high incidences of wave blocking by other vehicles and wave reflections on surrounding structures. Particularly, the provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
(2) A roadway communication system according to the invention for achieving the above object has an arrangement wherein a plurality of road transmission antennas are disposed at different places along a road and each radiate the same cell with waves carried at the same frequency and containing the same content, wherein a position marker is disposed at or near the road for indicating a position on the road at which reception levels of the waves radiated from the plural road transmission antennas are switched, and wherein a vehicle mounted device receiving the waves radiated from the road transmission antennas via vehicle reception antennas performs any one of the following operations in response to detection of the position marker at or near the road, the operations including switching of directivities of the vehicle reception antennas, and switching or combining the received signals (Claim 12).
In the roadway communication system, the position marker is disposed at place on the road or the like for indicating the position on the road at which the maximum reception levels are switched. In response to the detection of arrival of the vehicle at the position marker, the directivities of the vehicle reception antennas are switched, or the received signals are switched or combined. Accordingly, the processings become simpler than where the levels of the received signals are detected and compared.
It is noted that xe2x80x9cswitching the directivities of the vehicle reception antennas using phase controlxe2x80x9d means reception phase control for directivity change when an array antenna is used as the vehicle reception antennas.
(3) A roadway communication system according to the invention for achieving the above object has an arrangement wherein a plurality of road transmission antennas are disposed at different places along a road, each having a specific polarization characteristic and radiating the same cell with waves carried at the same frequency and containing the same content, and wherein a vehicle mounted device receiving the waves from the road transmission antennas performs polarization diversity reception (Claim 13).
According to the invention, the plural road transmission antennas radiate different polarization waves based on signals modulated with data of the same content. In this case, the differently polarized waves have different propagation characteristics and hence, are generally received by the vehicle on road at different field strengths. Therefore, if a wave of one polarization characteristic is blocked by the large vehicle, the vehicle mounted device is allowed to receive a wave of the other polarization characteristic. By switching the polarization characteristics of the vehicle reception antennas, the seamless communications between the road transmission antennas and the vehicle mounted device are ensured even if the wave incoming from one side is blocked.
The vehicle mounted device may detect reception levels of the waves received by the vehicle reception antennas on a polarization-characteristic basis so as to perform the diversity reception based on the reception level thus detected (Claim 14).
The reception level of the wave received by the vehicle reception antenna normally increases in value as the vehicle approaches the road transmission antenna radiating the wave. In a case where a wave to be received at the highest reception level is blocked by the large vehicle cutting in the transmission path thereof, for example, a reception level of another wave from a different road transmission antenna may become relatively higher than that of the former wave.
In consideration of this event, an arrangement may be made such that the reception levels of the waves received by the vehicle reception antennas are directly detected and the reception polarization characteristic is changed. Thus are attained more preferable communications.
The diversity reception may be accomplished by either of the following operations: an operation of switching or combining the signals which were received by the vehicle reception antennas and are to be decoded; and an operation of switching or combining the codes which were received by the vehicle reception antennas and then decoded.
The vehicle reception antennas may be a polarization array antenna, whereas the vehicle mounted device may further comprise reception-signal detection means for detecting a reception level or phase of the wave received by each of the vehicle reception antennas, so that the diversity reception means may perform the diversity reception using information on the reception level or phase detected by the reception-signal detection means.
This arrangement assumes a case where the array antenna or adaptive array antenna is applied to the vehicle reception antennas. The phase control of the antenna provides reception of a desired polarization wave.
The roadway communication system may further comprise a signal transmission unit for transmitting signals modulated with data of the same content to the road transmission antennas via a plurality of transmission lines, and may use an optical fiber radio signal transmission system as a transmission system for outputting the signals to the plural transmission lines. According to the invention, the signal transmission unit supplies radio frequency signals to the road transmission antennas through the optical fibers and therefore, no need exists for each road transmission antenna to have a signal transmission unit with a frequency converter. This results in a simplified configuration of the road transmission antenna.
Orthogonal Frequency Division Multiplex (OFDM) technique in which a guard time is provided at each symbol may be used as a data modulation technique. OFDM system resisting multipath effect is preferably applied to the inventive arrangement wherein the waves incident in different directions are received. Particularly, the provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
The signal transmission from the plural road transmission antennas to the same cell involves fear that a fractional difference in carrier frequency may occur between the transmission stations. It is known that OFDM is more susceptible to the degradation of transmission quality than other transmission systems. The optical fiber radio signal transmission system provides a highly effective and economical solution to this problem, ensuring that the carrier frequencies from the stations are completely matched.
A roadway communication system according to the invention for achieving the above object comprises a plurality of road reception antennas for receiving differently polarized waves from the vehicle mounted device, and has an arrangement wherein the plural road reception antennas each have a specific polarization characteristic and are so disposed as to provide directivity to the same cell, each performing diversity reception based on the signals received by the road reception antennas.
According to the invention, the vehicle supplies vehicle data to the stationary station. The vehicle mounted device radiates waves of different polarization characteristics via the vehicle transmission antennas. As a result, these waves are received by the road reception antennas. The waves of the different polarization characteristics have different propagation characteristics. Therefore, even if a wave of one polarization characteristic is blocked by the large vehicle, the road reception antenna is allowed to receive a wave of the other polarization characteristic. Thus is ensured the seamless communications between the vehicle mounted device and the road reception antenna.
The roadway communication system may further comprise reception-level detection means for detecting reception levels of the plural road reception antennas on a polarization-characteristic basis, and the diversity reception means may perform the diversity reception based on the reception level detected by the reception-level detection means.
In a case where the large vehicle moves toward a road transmission antenna to receive the polarized waves from the vehicle mounted device at the maximum reception level thereby to block the polarized waves, for example, a reception level of another road reception antenna at a different location may become relatively higher than that of the former reception antenna. In consideration of this event, an arrangement may be made such that the reception levels of the waves received by the road reception antennas are detected and the reception antenna to receive the wave is changed. Thus are attained more preferable communications.
The diversity reception means may perform either of the following operations for diversity reception: an operation of switching or combining the signals received by the road reception antennas; and an operation of switching or combining the codes which were received by the road reception antennas and then decoded.
The roadway communication system may further comprise a signal reception unit for receiving, via transmission lines, the signals received by the road reception antennas, and may use an optical fiber radio signal transmission system as a transmission system for outputting the signals to the transmission lines.
According to the inventive arrangement, the signals received by the road reception antennas may be outputted to the transmission lines as at high frequencies. A signal selection unit may readily compare the received signals at high frequencies. This results in a simplified configuration of the road reception antenna and signal selection unit.
The vehicle mounted device may use Orthogonal Frequency Division Multiplex (OFDM) modulation technique, as a data modulation technique, in which a guard time is provided at each symbol.
OFDM system resisting multipath effect is preferably applied to the inventive arrangement wherein the waves from the vehicle on road are received in an environment having high incidences of wave blocking by other vehicles and wave reflections on surrounding structures. Particularly, the provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
(4) A roadway communication system according to the invention for achieving the above object has an arrangement wherein a plurality of road transmission antennas are disposed at different places along a road, each antenna having a specific polarization characteristic and radiating the same cell with the waves carried at the same frequency and containing the same content, wherein a position marker is disposed at or near the road for indicating a position on the road at which reception levels of the waves from the road transmission antennas are switched, and wherein a vehicle mounted device comprises vehicle reception antennas having different polarization characteristics for receiving the waves radiated from the road transmission antennas, and marker detection means for detecting an arrival of the vehicle at the position marker, and performs any one of the following operations in response to the marker detection means detecting the arrival of the vehicle at the position marker, the operations including the switching of the polarization characteristics of the vehicle reception antennas, the switching of the received signals or codes, or the combining of the received signals or codes.
In the roadway communication system, the road marker is disposed at place, such as on the road, for indicating the position on the road at which the maximum reception levels are switched. In response to the detection of arrival of the vehicle at the position marker, the polarization characteristic of the vehicle reception antenna is changed. Accordingly, the processings become simpler than where the levels of the received signals are detected and compared.
It is noted that xe2x80x9cswitching the polarization characteristics of the vehicle reception antennas using phase controlxe2x80x9d means reception phase control for polarization-characteristic change when an array antenna is used as the vehicle reception antennas.
(5) A roadway communication system according to the invention for achieving the above object has an arrangement wherein a plurality of road transmission antennas are disposed at different places along a road and each radiate the same cell with OFDM-modulated waves containing the same content, and wherein a vehicle mounted device receives the waves radiated from the road transmission antennas and demodulates the received waves.
In the roadway communication system, the plural road transmission antennas radiate the waves based on signals OFDM-modulated with data of the same content. In this case, the waves from the road transmission antennas are incident on the vehicle on road along different directions. Therefore, when a wave incoming in one direction is blocked by the large vehicle, the vehicle mounted device is allowed to receive a wave incident thereon in another direction, the reception level of which becomes relatively higher than the former wave. This ensures the seamless communications between the road transmission antennas and the vehicle mounted device.
OFDM system resisting multipath effect is preferably applied to the inventive arrangement wherein the waves from the vehicle on the road are received in an environment having high incidences of wave blocking by other vehicles and wave reflections on surrounding structures. As a result, the communication quality is not degraded.
The roadway communication system of the invention may further comprise a signal transmission unit for transmitting signals modulated with data of the same content via a plurality of transmission lines to the road transmission antennas, and may use an optical fiber radio signal transmission system as a transmission system for outputting the signals to the transmission lines.
In the inventive arrangement, the signal transmission unit supplies the radio frequency signals to the road transmission antennas via the optical fibers and therefore, no need exists for each road transmission antenna to have a signal transmission unit with a frequency converter. This results in a simplified configuration of the road transmission antenna.
It is preferred that Orthogonal Frequency Division Multiplex (OFDM) modulation technique in which a guard time is provided at each symbol is used as a data modulation technique. The provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
A roadway communication system according to the invention for achieving the above object has an arrangement wherein a vehicle mounted device radiates OFDM-modulated waves via a vehicle transmission antenna, and wherein the plural road reception antennas are disposed at different places along a road as providing directivity to the same cell and each perform demodulation using signals received by the road reception antenna.
According to the invention, the vehicle supplies the vehicle data to the stationary station. In this case, the vehicle mounted device radiates the waves via the vehicle transmission antenna while the reception levels of the road reception antennas are affected by the multipath transmission. The invention employs the plural road reception antennas and OFDM system resisting multipath effect, thereby ensuring positive wave reception on the stationary station side and error-free data recovery.
In the roadway communication system, the road reception antennas may use an optical fiber radio signal transmission system for outputting the received signals to transmission lines to the road reception means. According to the inventive arrangement, the road transmission antennas supply the radio frequency signals to the road reception means via the optical fibers and therefore, no need exists for each road reception antenna to have a signal transmission unit with a frequency converter. This results in a simplified configuration of the road reception antenna.
The vehicle mounted device may use OFDM modulation technique, as a data modulation technique, in which a guard time is provided at each symbol. The provision of the guard time at each symbol is effective to obviate the inter-symbol interference associated with delay in the multipath transmission.
(6) The roadway communication system according to the invention for achieving the above object has an arrangement wherein the plural road transmission antennas each define an individual one of plural sub-areas which are constituting a single cell (Claim 12).
According to the invention, the waves modulated with road traffic data of the same content are individually incident on the respective sub-areas so that the incoming direction of the wave changes each time the vehicle transfers to the next sub-area. Accordingly, if the wave is blocked in one area, the vehicle moving to the next sub-area is allowed to receive the wave. This prevents the occurrence of a communication breakdown between the vehicle and the stationary station, ensuring the seamless communications.
Since the sub-area defined by each road transmission antenna is one fraction of a single cell, the road transmission antenna requires a small transmission power. This results in reduced cost for the road antennas.
In the case where the sub-areas are defined, as well, the waves of the same frequency are simultaneously incident on the vehicle mounted device along different directions. Accordingly, it is preferred for the vehicle mounted device to select the wave of the maximum reception level in order to avoid the fading effect.
(7) The roadway communication system according to the invention for achieving the above object has an arrangement wherein communications are carried out over a plurality of continuous cells, using signals at the same frequency and of the same content (Claim 13).
This arrangement provides the vehicle mounted device with the seamless communications free from frequency switching (handover) during the travel of the vehicle, also contributing to a simplified configuration of the vehicle mounted device.
(8) The roadway communication system according to the invention for achieving the above object has an arrangement wherein the plural road transmission/reception antennas are disposed near a cell boundary with respect to a longitudinal direction of the road (Claim 14).
In this arrangement, the road transmission antenna is located near a road transmission antenna of a neighboring cell and hence, the waves radiated from these transmission antennas are incident on the vehicle on road along different directions.